As disclosed in JP-A-8-338850 and JP-A-2000-310646, for example, known rotation sensors are configured to implement non-contact detection for detecting the revolution and/or the rotative direction of a gear made from a metallic material such as a ferrous material. The rotation sensor disclosed in JP-A-8-338850 includes a case located outside teeth or a gear. In the configuration of JP-A-8-338850, the case accommodates a magnet and a hall IC device including two hall elements. Each hall element sends a voltage signal according to a magnetic flux flowing between the magnet and the gear. The rotation sensor detects the revolution of the gear according to a differential output of two signals sent from the two hall elements.
FIGS. 12A to 12D show an operation of a comparative example of a rotation sensor having a configuration similar to those of JP-A-8-338850 and JP-A-2000-310646. In FIG. 12A, a contaminating particle 3 such as a magnetic foreign matter may be magnetically attracted by a magnet 20 to adhere on a case 10 of the rotation sensor. In such a state, the contaminating particle 3 is located in the place where the density of the magnetic flux between a tooth A of a gear 2 and the magnet 20 is strong. As illustrated in FIGS. 12B and 12C in this order, when the gear 2 rotates at a low revolution frequency, such as 100 Hz, the contaminating particle 3 moves on a surface 13 of the case 10 on the side of the gear 2 along with rotation of the gear 2. Subsequently, the tooth B of the gear 2 on the rear side relative to the rotative direction moves toward the contaminating particle 3. Thus, the density of the magnetic flux flowing between the tooth B of the gear 2 and the magnet 20 becomes stronger than the density of the magnetic flux flowing between the tooth A of the gear 2 and the magnet 20 through the contaminating particle 3. Consequently, as illustrated in FIGS. 12C and 12D, the contaminating particle 3 moves on the surface 13 of the case 10 toward the space between the tooth B of the gear 2 on the rear side relative to the rotative direction and the magnet 20. In the present state, the contaminating particle 3 passes through the space among hall elements 31 and 33 and the gear 2 to reduce the apparent space among the hall elements 31 and 33 and the gear 2. Consequently, the apparent magnetic resistance among the hall elements 31 and 33 and the gear 2 decreases. Thus, the hall elements 31 and 33 may send its detection signals at wrong output timings in this way to cause the rotation sensor to detect the revolution of the gear 2 higher than the actual revolution of the gear 2.
In another exemplified configuration, a rotation sensor shown in FIGS. 13A to 13D includes three hall elements 31, 32, 33. The rotation sensor detects the rotative direction and the revolution of the gear 2 according to the phase difference between a differential output of one group of the hall elements 31 and 32 adjacent to each other and a differential output of the other group of the hall elements 32 and 33 adjacent to each other. As illustrated in FIG. 13C and FIG. 13D, when the contaminating particle 3 passes through the space between the gear 2 and the hall element 32, which is located at the center of the rotation sensor, the hall element 32 may send its detection signal at a wrong output timing. Consequently, the phase difference between the differential output of the one group of the hall elements 31 and 32 and the differential output of the other group of the hall elements 32 and 33 may vary in this way. As a result, the rotation sensor may detect a wrong rotative direction of the gear 2 opposite to the actual rotative direction of the gear 2.